Shepherd, John T.

Venous occlusion plethysmography is a simple but elegant technique that has contributed to almost every major area of vascular biology in humans. The general principles of plethysmography were appreciated by the late 1800s, and the application of these principles to measure limb blood flow occurred in the early 1900s. Plethysmography has been instrumental in studying the role of the autonomic nervous system in regulating limb blood flow in humans and important in studying the vasodilator responses to exercise, reactive hyperemia, body heating, and mental stress. It has also been the technique of choice to study how human blood vessels respond to a variety of exogenously administered vasodilators and vasoconstrictors, especially those that act on various autonomic and adrenergic receptors. In recent years, plethysmography has been exploited to study the role of the vascular endothelium in health and disease. Venous occlusion plethysmography is likely to continue to play an important role as investigators seek to understand the physiological significance of newly identified vasoactive factors and how genetic polymorphisms affect the cardiovascular system in humans.

Human umbilical artery was perfused with Krebs-Ringer bicarbonate-glucose medium at 37 C and pH 7.4. An increase in transmural pressure elicited a vasoconstriction. The artery wall contained no significant amount of norepinephrine, which supports the anatomic evidence of absence of nerve supply. Thus, the response to stretch is due to a smooth muscle cell-borne positive feedback mechanism. The myogenic reflex could still be elicited at 44 C but was reversibly abolished by cooling to 4 C. It was reversibly abolished in the absence of calcium. K2SO4-Ringer solution (50 mm in K) markedly increased the tone of the artery after an initial vasodilatation and induced rhythmicity when the perfusion pressure was increased. Bradykinin, 5-hydroxytryptamine, and angiotensin caused vasoconstriction when the artery no longer responded to stretch. Stretch could induce vasoconstriction when angiotensin no longer elicited a response because of tachyphylaxis. The stretch reflex was not blocked by phenoxybenzamine which suppressed the response to 5-hydroxytryptamine. Tachyphylaxis was not observed with bradykinin; the effect of bradykinin was not blocked by phenoxybenzamine.

Following attempted denervation of the heart by the technic of regional neural ablation, dogs with incomplete cardiac denervation were shown to have the same supersensitivity to l-norepinephrine as dogs in which the denervation of the heart was complete. Dogs with chronic bilateral stellate ganglionectomy or those pretreated with reserpine had cardiac acceleration in response to the administration of tyramine or to stimulation of the stellate cardiac nerves, but did not demonstrate supersensitivity to l-norepinephrine. No supersensitivity was seen in dogs with chronic bilateral cervical vagotomy. Excision of the right stellate and caudal cervical ganglia and the immediately adjacent right vagus nerve resulted in supersensitivity to l-norepinephrine. In these animals cardiac acceleration resulted from stimulation of the left stellate cardiac nerves or from the administration of tyramine. The supersensitivity was lost after excision of the sinoatrial node. It is concluded that one can uniquely denervate the sinus node and that dogs so treated will develop supersensitivity to l-norepinephrine despite the presence of functional sympathetic nerves to the rest of the heart.

In three normal dogs and in six dogs with cardiac denervation, there was a rapid increase in cardiac output (electromagnetic flowmeter) from the start of exercise. The denervated group showed an average delay of 25 sec in reaching plateau values of cardiac output and a deficit of 18% in the accumulated flow over the first minute of running; augmentation of output was due principally to a rapid increase in stroke volume since acceleration of heart rate was slow. In normal dogs, the augmentation of output came from a rapid increase in heart rate. The steady-state values for cardiac output and oxygen consumption were similar in the two groups as was the pattern of oxygen consumption in the first minutes of exercise and the manner in which the oxygen debt was repaid. This suggested that the denervated dogs extracted more oxygen from the circulating blood in the first moments of running. The changes with exercise in aortic blood pressure were similar in the two groups.

Dogs with chronic cardiac denervation by the technic of regional neural ablation showed an unchanged capacity for work as measured by oxygen consumption. The relation of cardiac output to oxygen consumption during exercise remained unchanged from preoperational values. When the dogs started to run, the heart rate rose slowly over 1.5 min to reach a steady value proportional to the work performed. When exercise was stopped, the heart rate declined slowly. With mild exercise, the increase in cardiac output was mainly through stroke volume; with more severe exercise, increase in stroke volume and heart rate contributed equally, in contrast to the normal dog where the increase in rate predominates. Neither the pattern of the change in heart rate nor the plateau values were altered by adrenalectomy. The change in rate was not attributable to change in intravascular temperature. In an equivalent dose of base, norepinephrine caused tachycardia but little or no change in cardiac output, whereas epinephrine resulted in an increase in cardiac output with but a modest increase in heart rate.

The effect of head-down neck flexion on forearm and calf blood flow was determined in 10 healthy male subjects. The subject lay prone, with the neck slightly extended and the chin resting on a soft-padded support at the edge of the table. The chin support was then removed, and the subject maximally flexed and lowered the neck. This was followed by return to the initial position. Neck flexion caused a rapid decrease in blood flow in both forearm and calf; at 30 s this averaged 39 and 35%, respectively. The flow in both forearm and calf gradually recovered as the neck flexion was sustained and approached the control values at the end of 10 min. The blood flow at the ankle was unchanged, indicating that the decrease occurred in the skeletal muscles. The arterial blood pressure and heart rate were unchanged; thus the decrease in flow was due to vasoconstriction. The fact that the decrease was evident as soon as the head was lowered indicated that it was nervously mediated. Neither contraction of the flexor muscles of the neck nor venous congestion of the head, in the absence of the head-down position, altered the blood flow. Although the mechanism of the decrease in flow has not been determined, the studies demonstrate that in response to certain stimuli, the resistance vessels in the skeletal muscles of the forearm and calf undergo a similar nervously mediated vasoconstriction.

The purpose of this study was to determine whether prolonged unloading of cardiopulmonary baroreceptors with lower body negative pressure (LBNP) causes constant increases in sympathetic outflow to skeletal muscles. Eight healthy subjects underwent a 20-min control period followed by 20 min of 15-mmHg LBNP. This pressure was selected because it did not cause any significant change in mean arterial blood pressure (sphygmomanometry) or heart rate, suggesting that the cardiopulmonary baroreceptors were selectively unloaded and the activity of the arterial baroreceptors was unchanged. Muscle sympathetic nerve activity in the peroneal nerve (MSNA, microneurography) increased from an average of 21.8 +/- 1.7 bursts/min over the last 5 min of control to 29.0 +/- 2.9 bursts/min during the 1st min of LBNP (P less than 0.05 LBNP vs. control). The increase in MSNA observed during the 1st min was sustained throughout LBNP. Forelimb blood flow (plethysmography) decreased abruptly at the onset of the LBNP from a control value of 4.3 +/- 0.5 ml.min-1.100 ml-1 to 2.5 +/- 0.2 at the 1st min; the flow then increased and remained significantly above this value, but below the control value, throughout LBNP. Similar blood flow findings were obtained in additional studies, when the hand circulation was excluded during the flow measurements. Forearm skin blood flow (laser Doppler) also decreased abruptly at the onset of LBNP and was followed by partial recovery, but these changes were too small to account for all the increases in limb blood flow over the course of LBNP.(ABSTRACT TRUNCATED AT 250 WORDS)

5-Hydroxytryptamine (serotonin) was infused into anesthetized dogs at a rate of 20 µg/kg/min. In nine sets of observations on three dogs the increase in the difference of pressure between the pulmonary artery and the left atrium, which averaged 55%, consistently exceeded the increase in pulmonary blood flow, which averaged 16%. 5-HT therefore is a potent constrictor of pulmonary vessels, even in small concentrations. No changes in the pulmonary-artery wedge and pulmonary-vein pressures were detected during the infusions of 5-HT, nor was there any change in the volume of blood between the pulmonary artery and the root of the aorta. With this dose of 5-HT the principal site of the increased resistance to flow through the lungs appeared to be in the precapillary vessels. In the isolated perfused lung, moderate constriction of pulmonary veins also was produced by large doses of 5-HT.

Ten normal subjects performed a 90-s isometric exercise [20, 30, and 40% of maximal voluntary contraction (MVC) of the flexor muscle of the right index finger or quadriceps muscle of the right leg. Contralateral forearm and calf blood flows (strain gauge plethysmography) and arterial blood pressure (auscultation) were measured simultaneously. Each exercise caused a decrease in forearm vascular resistance and a progressive increase in calf resistance. These changes were greatest with the 40% MVC. With finger exercise at 20 and 40% MVC, the percentage decreases in forearm vascular resistance from control were 12.3 and 22.7%, respectively (P less than 0.01). Similar decreases (9.5 and 24.9%, respectively; P less than 0.01) were noted with exercise of the quadriceps muscle. By contrast, the corresponding increases in calf vascular resistance were greater (P less than 0.01) with quadriceps exercise (13.3 and 55.4%, respectively) than with finger exercise (6.0 and 36.0%). Arrest of the circulation to the exercising muscles just before the exercise ended caused an abrupt increase in forearm vascular resistance and a decrease in calf resistance. These studies provide further evidence of the heterogeneity of responses of forearm and calf resistance vessels to certain cardiovascular stimuli.

Simultaneous measurements were made of changes in vascular resistance in the forearm and calf in response to moving from supine to sitting or to head-down tilt. The subjects were healthy male volunteers, 21–63 yr. Blood flows were measured by venous occlusion plethysmography using mercury-in-Silastic strain-gauges. The gauges were maintained at the same level relative to the heart during the postural changes. Arterial blood pressure was measured by auscultation; heart rate was counted from the plethysmograms. Changing from supine to sitting caused a decrease in forearm blood flow from 4.13 +/- 0.14 to 2.16 +/- 0.19 ml.100 ml-1.min-1. Corresponding calf flows were 4.21 +/- 0.32 and 4.40 +/- 0.59 ml.100 ml-1.min-1. There was no change in mean arterial blood pressure, and heart rate increased by 8.0 +/- 1.5 beats/min. Arrest of the circulation of both legs with occlusion cuffs on the thighs before sitting, to prevent pooling of blood in them, reduced the degree of forearm vasoconstriction. Neck suction (40 Torr) during sitting, to oppose the decrease in transmural pressure at the carotid sinuses, inhibited the vasoconstriction. During a 30 degrees head-down tilt, there was a dilatation of forearm but not of calf resistance vessels. A Valsalva maneuver caused a similar constriction of both vascular beds. Thus, when changes in vascular resistance in forearm and calf are compared, the major reflex adjustments to changes in posture take place in the forearm.